The present invention relates to a hollow body made of plastic of multi-layer polymeric structure.
Hollow bodies made of plastic of multi-layer polymeric structure have been developed to meet usage requirements which demand properties which cannot be conferred by a single thermoplastic. This technique has been used in particular when the issue is one or manufacturing hollow bodies made of plastic which have both high rigidity at ordinary temperature and good impermeability to liquids and gases they are intended to contain. In this case, the impermeability function is generally provided by a layer internal to the structure, of very small thickness and low mechanical strength, which is made of a material which behaves like a barrier to the liquids and gases contained in the hollow body.
One technique for producing hollow bodies made of plastic of multi-layer polymeric structure currently consists in assembling two or more elements of similar multi-layer structure, by welding.
We are also witnessing a significant reduction in the permissible amounts of liquid and vapour which can escape into the environment from containers containing organic substances. In the field of fuel tanks, new standards imposing extremely low levels of permissible losses will very soon come into force.
In hollow bodies manufactures as described above by assembling welded multi-layer elements, the region of the welding planes has lessened impermeability, given the crushing of the multi-layer structure in the plane of welding, which usually causes layers of one element to be folded down onto that of the welded element and the internal layer of each element to be welded to that of the other element. This then generally results in a discontinuity in the barrier layer of the structure of the hollow body produced, giving rise therefore to a preferred path for liquid and vapour leakages.
Japanese Patent Application No. JA-63/178931, which describes joining planes resulting from the assembly of multi-layer structures in which the ends of the barrier layers meet at a point inside the welded seam, is known.
The impermeability obtained with such joints is not, however, yet good enough to comply with the very low limits imposed by the new standards, given the difficulties there are in maintaining contact between the end or the barrier layers of each element along the entire length of the run of welding.